Serve a cache hit with no database read or write (closes #309)

A hit opens the variant file and nothing else: its content type comes
from the file's first bytes, and a file in no output format is a miss.
Its access time and hit count, and the access time of a cached source a
miss uses, go without waiting to the goroutine that wrote late counts,
which now writes them all once a second; each kind of access time waits
in a channel of 10,000 that drops the rest. checkNegativeCache leaves
expired rows to CleanExpired. The .meta files, metaCache, Lookup and
golang-lru go, with their tests. Dropping the .meta skip made the
variant and source reconciliation walks identical, so they share one.
make bench runs a parallel cache hit benchmark on the host, outside
make check.

Model: opus-5-5

internal/imgcache/service_internal_test.go

@@ -2,7 +2,6 @@ package imgcache
 
 import (
 	"context"
-	"encoding/json"
 	"io"
 	"os"
 	"testing"
@@ -552,10 +551,10 @@ func TestService_Get_Caching(t *testing.T) {
 // a variant of it and sets back two days every time pixa stored for them.
 // For the source: fetched_at and last_accessed_at in source_content, and the
 // access and modification times of its file. For the variant: created_at and
-// last_accessed_at in variant_content, the created_at in its .meta file, and
-// the access and modification times of its file and of its .meta file. It
-// then checks that the variant is still served from the cache and that a new
-// variant is made from the cached source without fetching it again.
+// last_accessed_at in variant_content, and the access and modification times
+// of its file. It then checks that the variant is still served from the
+// cache and that a new variant is made from the cached source without
+// fetching it again.
 //
 // Left at the present: the files' status change and birth times, which a
 // program cannot set, and the times of the directories holding the files,
@@ -598,6 +597,8 @@ func TestService_Get_ServesSourceAndVariantCachedTwoDaysAgo(t *testing.T) {
 
 	_ = resp.Content.Close()
 
+	waitForPendingCountsWritten(t, svc.cache)
+
 	fetches := readCacheStatsCounters(t, svc.cache).upstreamFetchCount
 	if fetches != 1 {
 		t.Errorf("upstream fetches = %d, want 1", fetches)
@@ -636,41 +637,16 @@ func backdateVariant(
 	t.Helper()
 
 	key := imagerequest.CacheKey(req)
-
-	content, size, contentType, err := cache.variants.LoadWithMeta(key)
-	if err != nil || contentType == "" {
-		t.Fatalf("reading the variant's .meta file: content type %q, error %v",
-			contentType, err)
-	}
-
-	_ = content.Close()
-
-	meta, err := json.Marshal(VariantMeta{
-		ContentType: contentType,
-		Size:        size,
-		CreatedAt:   when.Unix(),
-	})
-	if err != nil {
-		t.Fatalf("encoding the variant's .meta file: %v", err)
-	}
-
 	path := cache.variants.keyToPath(key)
 
-	err = os.WriteFile(path+variantMetaSuffix, meta, StorageFilePerm)
-	if err != nil {
-		t.Fatalf("writing the variant's .meta file: %v", err)
-	}
-
 	updateOneRow(t, cache,
 		`UPDATE variant_content SET created_at = ? WHERE cache_key = ?`,
 		when.UTC().Format(sqliteTimestampFormat), string(key))
 	setVariantLastAccessed(t, cache, key, when)
 
-	for _, file := range []string{path, path + variantMetaSuffix} {
-		err = os.Chtimes(file, when, when)
-		if err != nil {
-			t.Fatalf("setting back the times of %s: %v", file, err)
-		}
+	err := os.Chtimes(path, when, when)
+	if err != nil {
+		t.Fatalf("setting back the times of %s: %v", path, err)
 	}
 }